Typically, a disk drive has a circular magnetic disk for storing data. The disk is mounted on a spindle which is rotated about a longitudinal axis or axis of rotation. Data is read from and written to the surface of the disk while the disk is rotating. To rotate the disk, a spindle motor transmits a driving force to rotate the spindle. A mechanism then positions a magnetic head over a desired area of the disk surface to read data already recorded on the disk or to write data to the disk. To position the head accurately, it is very important that the center of the disk be precisely aligned with the spindle to prevent the disk from being offset during rotation. Therefore, a process is needed to align the center of the disk with the axis of rotation of the spindle. In other words, a process is needed to keep the offset of the disk within an allowable error (tolerance), and fix the disk to the spindle during manufacturing.
The operation of adjusting the offset of a rotating disk to be within an allowable error is generally referred to as the adjustment of rotation balance. Rotation balance has two components: static balance and dynamic balance. Static balancing refers to decreasing the vibration component generated by a translational force during rotation. Dynamic balancing refers to decreasing the vibration component generated by torque during rotation. Generally, if the distance between a plurality of masses fixed to an axis of rotation or spindle is short, the effect of the dynamic imbalance is negligible relative to the effect of the static imbalance. However, as the distance increases, the effect of the dynamic imbalance increases; and the dynamic imbalance can no longer be ignored.
In a hard disk drive, when a small number of disks are fixed to the spindle, the length of the spindle is small; therefore, the dynamic balance may be ignored. However, as the number of disks increases, both the dynamic and static balance need to be adjusted. Preferably, the amount of static and dynamic imbalance is 0. However, in practice, it is very difficult to completely eliminate all imbalance. Therefore, a process is needed to adjust the static and dynamic balance during disk drive manufacture so that the amount of imbalance falls below a predetermined threshold value to prevent problems when the disk drives are used. The adjustment of the static and dynamic balance will be described below.
Adjusting Static Balance
Initially the adjustment of the static balance will be described while referring to FIGS. 1, 2a, 2b and 2c. FIG. 1 is a diagram of the force resulting from disk offset. A magnetic disk drive 200 has a disk 204, e.g. a magnetic disk, fixed to a spindle 202. Because the outer diameter of the spindle 202 is smaller than the inner diameter of a central hole in the disk, a gap as large as several hundred microns forms between the disk and the spindle. Accordingly, when the disk 204 is fixed to the spindle 202, the center of the spindle of the disk 0 is frequently offset, by amount e, from the center of gravity G of the disk, therefore the disk is frequently imbalanced. If the disk is imbalanced, a translational force P is generated when the disk 204 rotates.
FIGS. 2a, 2b and 2c show the relationship between the periodic motion of the disk and the direction of the translational force resulting from the offset. The translational force P, with a continuously changing direction, is exerted between the disk 204 and the spindle 202. As the amount of vibration from the translational force P increases, errors are more likely to occur when reading or writing data.
Referring back to FIG. 1, a disk 204 of mass m rotates at angular velocity .omega. while offset by dimension e relative to the center O of the spindle 202 of a fixed disk drive 200. The translational force P has a vertical component F urging the disk drive 200 downward. The magnitude of the vertical force F is determined by the following equation (1): EQU F=me.omega.2 sin(.omega.t) (1)
For example, if a 2.5 inch aluminum disk rotates at 3,600 revolutions per minute (RPM) with a offset of 0.1 mm, the maximum value of the magnitude of force F is about 10 g. Equation (1) shows that the magnitude of force F is directly proportional to both the rotation frequency and the number of disks. If either the rotation frequency or the number of disks increases, then the magnitude of force F will increase. For example, if the 2.5 inch aluminum disk above is rotated at 4,800 RPM, the magnitude of force F is 1.8 times larger than at 3,600 RPM. The static balance adjustment means decreasing the amount of vibration caused by the translational force during disk rotation.
Adjusting the Dynamic Balance
Next, the adjustment of the dynamic balance will be described while referring to FIGS. 3a and 3b. FIG. 3a shows a plurality of disks 204 fixed to a spindle 202. FIG. 3b is a dynamic model of the plurality of disks of FIG. 3a. In FIG. 3b, according to rigid body kinematics, the rotation imbalance of a distributed mass system can be equivalently represented by two concentrated weights 201, 203 on two arbitrarily selected planes S1, S2 on the axis 202. The two concentrated weights 201, 203 have mass m1 and m2, respectively, and have corresponding vectors r.sub.1 and r.sub.2. The planes S1, S2 are spaced apart at a distance 1 along the spindle 202. During rotation, a vibration component is generated by the torque based on the equivalent amount of imbalance m.sub.1.multidot.r.sub.1, m.sub.2.multidot.r.sub.2 between the two planes. Adjusting the dynamic balance decreases the amount of vibration from the torque.
To satisfy the increased demand for high speed and large storage capacity, disk drives have more disks and rotate faster. Consequently, the problem of efficiently reducing the imbalance has become more significant. Therefore, the adjustment of static balance and dynamic balance has become increasingly important in disk drive manufacture.
Using a conventional technique, imbalance is adjusted by: rotating a disk, measuring the position and amount of imbalance, and attaching a counterweight with a controlled mass to a side opposite a stopped position. The counterweight must be attached while the disk is stopped. Furthermore, the balance adjusting operation must be repeated many times for accurate adjustment. Because the disk is repeatedly rotated and stopped, the method has the disadvantage of taking a long time.
Another method has been proposed which uses a screw as a counterweight. However, this method not only had the disadvantages discussed above, but also had the disadvantage of being difficult to make a fine balance adjustment since the tapped hole has a predetermined position.
In addition, Japanese Published Unexamined Patent Application No. 60-187966 discloses a method of rotating the disk at a frequency higher than a critical speed while the disk is offset and fixing the disk to the axis when the rotation is brought close to 0. However, rotating the disk above the critical speed puts a very large load on the spindle motor and may reduce the life of the disk drive. Therefore, this method is not always practical.
Recently, Japanese Published Unexamined Patent Application No. 3-69060 has disclosed a method using an offset compensator to overcome the disadvantages discussed above. The offset compensator has several components. A clamp, such as a magnet, is capable of moving radially across a disk. A spindle motor part guide is formed to enable the spindle motor to move radially with the clamp. A device measures the amount of offset between the center of a concentric circular or spiral track on the disk and the center of rotation of the spindle motor part using a push-pull and track-crossing signal from an optical pick-up. A driving device exerts acceleration on the spindle motor part. Acceleration is exerted on the spindle motor part and the clamp position of the disk is shifted using the inertia of the disk so that the center of the concentrically circular or spiral track is aligned with the center of rotation of the spindle motor part.
The offset compensator aligns the center of a concentric circular or spiral track of the disk with the center of rotation of the motor using the optical pick-up. In other words, by assuming that the center of gravity of the disk coincides with the center of a track, the center of the track is aligned with the axis of rotation of the spindle. Generally, the recording density of a fixed disk is much higher than a removable disk. Therefore, the alignment of the fixed disk requires much more accurate positioning than the disclosed removable disk. Since the center of a concentric circular track on the disk surface does not always exactly coincide with the center of gravity, the method may not provide satisfactory accuracy when used to align fixed disks having a high recording density. In fact, Publication No. 3-69060 discloses a method for aligning removable disks. However, the Publication does not describe a method of a aligning a fixed disk. Furthermore, the Publication describes a method of adjusting static balance and does not mention the adjustment of dynamic balance, which is needed when a plurality of disks are fixed to a spindle. In addition, the method also has the disadvantage that a track must be formed on the disk surface prior to the adjustment.